FIG. 1 illustrates a conventional automotive air conditioning system. The system includes compressor 1, having a variable displacement mechanism, condenser 2, receiver dryer 3, thermostatic expansion valve 4 and evaporator 5 serially connected. The output of evaporator 5 is connected to the input of compressor 1. Thermostatic expansion valve 4 controls the flow volume of the refrigerant which flows into evaporator 5. The operation of thermostatic expansion valve 4 is dependent upon the temperature of the refrigerant which flows out of evaporator 5.
Among the drawbacks associated with such prior art systems are, for example, the air conditioning load is extremely small and the compression ratio between the inlet and outlet port of the compressor also may be small. As a result, the quantity of refrigerant circulated in the system is very small. In turn, such circulation may give rise to various problems relating to lubrication, control of thermodynamic properties at the evaporator and evaporator cooling efficiencies discussed hereafter.
Lubrication oil normally is suspended in the refrigerant. Accordingly, a decrease in the quantity of circulated refrigerant decreases the quantity of lubrication oil circulated in the compressor. If after a period of such relatively low lubricant circulation, the automobile is driven at a relatively high speed wherein the rotational speed of compressor 1 correspondingly increases to a relatively high value, driving parts in compressor 1 may be damaged due to insufficient lubrication during the transition.
Further, in an automobile air conditioning system which includes fixed throttle valve 6 as a decompression mechanism and accumulator 7 at the outlet of evaporator 5 as shown in FIG. 2, inadequate lubricant circulation may occur. Specifically, lubrication oil which flows into accumulator 7 and accumulates therein, does not readily flow therefrom due to the decrease in the quantity of refrigerant circulating in the system. Accordingly, during stages when the rotational speed of compressor 1 is below its upper range, the driving parts in compressor 1 also may be damaged due to insufficient lubrication.
In addition, if the quantity of refrigerant circulated in the system is extremely small, the temperature change in the refrigerant at the outlet port of evaporator 5 also is small. Accordingly, the response to such small temperature changes in refrigerant temperature, which are detected at detecting portion 41 of thermostatic expansion valve 4 to control the position of valve 4, is very slow. Accordingly, it is difficult to control the valve so as to stabilize its position. As a result, the evaporating pressure and superheat at evaporator 5 are unstable. In turn, the temperature of the air which is conditioned by evaporator 5, varies in accordance with the above evaporator thermodynamic instability and thus reduces comfort in the passenger compartment.
Further complications may arise in an air conditioning system which incorporates an evaporator having a plurality of conduits. In such systems, the refrigerant is not effectively distributed into each conduit when the volume of refrigerant circulated in the system is small. Specifically, only gas-state refrigerant flows into particular conduits. Because liquid-state refrigerant does not flow into those conduits, the portion of the evaporator in which those conduits extend virtually does not contribute to cooling. As a result, the cooling efficiency of the evaporator decreases. Accordingly, small air conditioning load may produce low evaporator cooling efficiencies.